U.S. Pat. No. 5,282,718 entitled "Case Treatment for Compressor Blades, granted to Steven G. Koff (coinventor of this patent application) and others on Feb. 1, 1994 and assigned to United Technologies Corporation the assignee common to this patent application, discloses and claims a vaned passage treatment for the compressor blades of the compressor rotor. When applied to the compressor rotor the vaned passage is disposed in the outer air seal or shroud that surrounds the rotating blades. The vaned passage at this location is in a flow field that has different characteristics than the characteristics of the flow field adjacent to the hub. The vaned passage treatment in the shroud surrounding the rotor hub, which is a stationary component of the engine, and the vaned passage treatment of the hub, which is a rotating component, will evidence a different loading pattern and have different stress considerations.
Additionally, and of paramount importance is that the blading geometry and the flow field associated with a rotor tip are very different from those of a stator hub in a modern aircraft engine compressor. These differences are demonstrated in the table presented immediately hereinbelow.
______________________________________ ROTOR TIP STATOR HUB ______________________________________ Camber angle low high Blade stagger angle ref. high low to axial direction Blade corner stalls no yes Inlet relative Mach No. high moderate ______________________________________
This invention to some extent relates to the technology disclosed in U.S. Pat. No. 5,282,718 in as much as both utilize vaned passages where the prior art teaches the use of vaned passages formed in a fixed component and the present invention relates to the vaned passages formed in a rotating component. As will become more evident from the description to follow hereinbelow these vaned passage treatments present their individualized problems and the use of vaned passage hub treatment is not an obvious extension of the vaned passage treatment disclosed in the prior art. As for example, in the rotating blade environment, the vaned passage treatment is merely added to the surrounding wall of the blades which is an existing engine component. When applied to an engine where hardware that can be modified to accommodate the vaned passage treatment is not available, such a configuration is not suggestive of utilizing vaned passage treatment in as much as extensive changes to the engine itself need to be made in order to accommodate such treatment. As for example, if the stator vanes are shrouded on the inner diameter, the entire stator vane, namely, the outer shroud attachment, the airfoil, and the structural details need to be changed due to changes in loading and stresses.
As one skilled in this technology realizes, compressor and fan stalls of axial flow gas turbine engines or rotating machinery typically initiate at the endwalls especially for the modern highly loaded compression systems. As described in the U.S. Pat. No. 5,282,718, supra, stalls that initiate in the rotor tip section of the fan or compressor can be delayed with the application of vaned passage casing treatment. The vaned passage casing treatment at this location substantially delays the onset of stall without impacting the component efficiency and the overall engine operating performance.
Hub treatment reduces weight and complexity of the compressor section of the engine, and hence, the overall engine by eliminating variable geometry stators and the number of stages in the compressor section. This also results in costs savings.
The vaned passage hub treatment of this invention will significantly delay stall that has the propensity of occurring at the stator hub sections of the high pressure compressor. The vaned passage treatment of this invention is designed to avoid penalties to the compressor efficiency. As noted above, the vaned passage hub treatment of this invention can eliminate variable vane stages in engines that utilize these types of vanes to meet operability requirements, thereby reducing weight and cost while improving reliability, durability and maintainability. The use of this type of treatment to the hub can also increase pressure ratio per compression stage which would decrease the length, weight, total number of components and complexity of the overall engine.
This invention contemplates in one of its embodiments casting the vaned passage hub treatment in a single unit. In another embodiment this invention contemplates fabricating the vaned passage hub treatment by machining the turning vanes integrally with the full ring hub. Fabricating the cap from a full ring that serves to house the outer diameter of the vaned passage which also defines the inner diameter boundary for the main flowpath of the engine's working fluid. A third full ring that is formed into a support member that supports and allows the installation of the cap. These three rings are diffusion bonded together to form the full hub assembly that is attached to the preceding rotor disk and is mounted contiguous with the tips of the vanes of a cantilevered mounted stator vane construction.
In still another embodiment, the hub is fabricated into a rotating disk assembly to be mounted in close proximity to the cantilevered stator row of vanes. The turning vanes are machined integrally into the rotating disk assembly. A cap formed from segmented arcuate members assembled end to end form together with the disk the treated passage. The cap defines the bottom boundary of the gas path of the engine's working fluid.
In all of the embodiments described hereinabove, the rotating-stationary relationship between the stator vane end and the hub shroud is required to 1) have a significant driving force for the gas path air to recirculate through the vaned passages and 2) be able to selectively siphon low momentum air from the main stream high momentum air in the gas path which reduces the opportunity to recirculate air unnecessarily and avoids efficiency penalties and excessive flow heating.
This invention contemplates coating the surface of the hub that is adjacent to the tips of the stator vanes with an abrasive material which grinds away the stator vane tips if these stator vanes should contact this surface while in operation. This is to assure that there will be a tight clearance between the vane ends and the hub contour.